The present invention relates to a method of concentrating various sludges such as waste liquor from pulp production.
The waste liquor from pulp production must be concentrated to a dry-substance content of greater than 55% before combustion in a combustion chamber of a soda recovery system is satisfactory possible. The heat contained in the flue gases resulting from the combustion have previously only been used for the production of steam.
Many soda recovery boilers, however, have been designed for a final concentration by direct flue gas evaporation in a flue gas evaporatory. The reasons for doing this include: (1) direct flue gas evaporation does not require expensive apparatus; (2) the evaporation plant itself may be smaller in size; and (3) some waste sludges form heavy deposits on heat surfaces in dry-substance contents of over 50% which means that they are very difficult to treat in indirect evaporators.
Flue gas evaporation, however, also has a number of drawbacks, including: (1) evaporation taking place in one stage and the secondary heat not being utilizable; (2) problems with odors and emissions; and (3) low steam production efficiency of the boiler.
For these reasons, many recovery boilers built according to the flue gas evaporation principle have been modernized by removing the flue gas evaporator, providing a new preheated for water for final cooling of the flue gases, and by building a further evaporating unit for final concentration of the liquor. Such modernization can sometimes be quite extensive, and in the case of heavily contaminating liquors, such as bagasse soda liquor, modernization is not possible at all.